Thermal Management

Thermal management is a significant challenge due to the increasing heat generation rates in commercial electronics, energy (solar cells, fuel cells, etc.), space, and defense systems. In many of these applications, thermal management is the bottleneck towards achieving optimal performance.

The DRL seeks to develop novel thermal management solutions to meet the significant demands in these various applications.

Thin-film evaporation -- With the ever increasing cooling demands of advanced electronics, thin film evaporation has emerged as one of the most promising thermal management solutions. High heat transfer rates can be achieved in thin films of liquids due to a small conduction resistance through the film to the evaporating interface. The center image shows a test fixture for studying thin film evaporation from nanoporous membranes inside of an environmental chamber, in which pressure, temperature, and contamination can be controlled. In thin film evaporation, maintaining a stable liquid film to attain high evaporation rates is challenging. We investigated nanoporous anodic aluminum oxide (AAO) membranes to supply liquid to the evaporating surface via capillarity. In this work, we achieved enhanced experimental control via the creation of a hydrophobic section within the nanopore. By creating a non-wetting section, the liquid is confined within the membrane to a region of well-controlled geometry. This non-wetting section also prevents flooding, where the formation of a thick liquid film degrades device performance (schematic (a)). Imaging of light transmitted through the membrane allows us to visualize the flooded surface during experiments (b). In order to control the wetting location, the pores are filled with photoresist, which is then etched a desired distance into the membrane pores. Figure (c) shows the top view of an AAO membrane and cross section views of membrane pores filled with photoresist with different etch depths are shown in (d)-(f). The top section exposed by the etch is then made hydrophobic via the deposition of a silane coating while the rest of the pore remains hydrophilic due to the photoresist. The remaining photoresist is removed, and a biphilic membrane is created (g). When heat flux is applied to the membrane surface, the liquid wicks into the membrane from the bottom and becomes pinned at the onset of the hydrophobic layer. As a result, the wetting in the membrane is controlled, flooding is prevented, and a stable evaporating surface in achieved. An optical image of a non-flooded biphilic membrane is shown in (h) and a schematic of the wetting in (i), where orange denotes the hydrophobic coating. With this approach, thin film evaporation from nanoporous media can now be studied for varying parameters such as pore size, porosity, and location of the meniscus within the pore.

Hotspot Thermal Management -- Thermal management is a primary design concern for numerous power-dense equipment such as power amplifiers, solar energy convertors, and advanced military avionics. During operation, these devices generate large amounts of waste heat (> 1 kW/cm2) from sub-millimeter areas. These concentrated heat loads are spatially and temporally non-uniform and cause hotspots which are localized regions with extreme heat flux and exceedingly high temperature that can adversely impact device performance and reliability.

In Device Research Laboratory, we study phase-change-based thermal management techniques targeted for cooling hotspots. Our approach uses capillary-fed thin-film evaporation. We experimentally characterized hotspot cooling with a silicon microstructured device via thin film evaporation in the absence of nucleate boiling and dissipated ultra high heat fluxes (6.0 kW/cm2) from a 620x640 µm footprint when the hotspot temperature was 290 °C. The average temperature over the entire 1x1 cm2 microstructured evaporative area as well as the local temperatures within a 3 mm radius from the hotspot were significantly lower (< 50 °C) than the hotspot temperature, indicating significant temperature gradient in the vicinity of the hotspot due to reduced thermal conductivity of silicon at high temperature.

Experimental results show that the capillary-limited dryout heat flux decreases by creating concurrent hotspots over the 1x1 cm2 microstructured area as well as by superposing moderate uniform background heating with the hotspot. Despite the decrease in the dryout heat flux, the total heating power dissipated via thin-film evaporation increases by spatially distributing the hotspots over the microstructured surface and by superposing mild uniform background heating with the hotspot. We also observed that the dryout heat flux is insensitive to the location of the hotspot, i.e., the dryout heat flux remained within the experimental error when the hotspot was created at different locations within the microstructured surface. We attribute this to the two-dimensional fluidic transport within the micropillar wick in our device.

In addition to experimental characterization, in Device Research Laboratory, we are engaged in developing theoretical models. We developed a semi-analytical thermal-fluidic model that compares reasonably well with our experiments. The model captures both the fluidic and thermal transport within the micropillar wick. Our experimental characterization and modeling highlight the promise of thin-film evaporation as a viable thermal management strategy for dissipating highly localized extreme heat fluxes from sub millimeter areas from high performance power electronics and radio frequency devices, where dissipating ultra-high heat fluxes is a significant challenge.

One project is developing a high performance air-cooled heat sink in collaboration with Prof. John G. Brisson in Mechanical Engineering, Prof. Jeffrey H. Lang in Electrical Engineering and Computer Science and Lockheed Martin. The Pumped Heat Exchanger (PHUMP) seeks to dissipate 1000 W with using only 33 W of electrical power with a thermal resistance of 0.05 K/W in a 4" x 4" x 4" volume. We are incorporating a multiple-condenser loop heat pipe, a blower with multiple impellers and a low-profile motor to achieve these metrics. A loop heat pipe is an enclosed two-phase system with an inherently high effective thermal conductivity due use of the high energy of vaporization. The working fluid evaporates where the heat pipe is in contact with the heat source and condenses where convective cooling occurs. By integrating the blower impellers along each wall of the condensers, high convective heat transfer is achieved with small air flow velocities and therefore low pumping power. This research is supported by DARPA, under the MACE program.